Electrical vacuum pump



1966 R. ZAPHIROPOULOS 3,231,175

ELECTRICAL VACUUM PUMP 2 Sheets-Sheet 1 Original Filed June 16, 1958 INVENTOR.

Renn Zaphiropoulos flit Attorney Fig.4

1966 R. ZAPHIROPOULOS 3,231,175

ELECTRICAL VACUUM PUMP 2 Sheets-Sheet 2 Original Filed June 16, 1958 INVENTOR. Renn Zaphiropoulos Attorney United States Patent O 3,231,175 ELECTRICAL VACUUM PUMP Renn Zaphiropoulos, Los Altos, Califi, assignor to Varian Associates, Palo Alto, Calif., a corporation of California Original application June 16, 1958, Ser. No. 742,274, now

Patent No. 3,018,944, dated Jan. 30, 1962. Divided and this application Nov. 9, 1961, Ser. No. 151,339

17 Claims. (Cl. 230-69) This application is a division of my co-pending application Serial No. 742,274, filed June 16, 1958, for improvements in electrical vacuum pump apparatus, which application has now matured into US. Patent 3,018,944.

The present invention relates in general to electrical vacuum pumps and more specifically to a novel electrical high vacuum pump of the type wherein cathode members of a reactive material are bombarded with high speed ions to disintegrate the reactive cathode material, the disintegrated cathode material condensing upon surfaces within the apparatus and there serving to getter gas molecules coming in contact therewith. Electrical vacuum pumps, using the aforementioned principle of operation, have become known in the art as getter ion vacuum pumps. Such pumps are extremely useful for providing uncontaminated high vacuums as required in many devices such as, for example, vacuum tubes, linear accelerators, electron microscopes, ammonium masers and the like.

Heretofore getter ion vacuum pumps have been constructor having a unitary cellular anode disposed between and spaced apart from two cathode plates. The unit was immersed in a strong magnetic field directed perpendicularly to the cathode plates and substantially coaxially of the cells of the anode. The anode was operated a few thousand volts more positive than the cathode plates resulting in the establishment of a glow discharge between the anode and the cathode plates whereby the cathode plates were bombarded with high speed ions there- 'by dislodging portions of the reactive cathode material. The disintegrated cathode material condensed upon the large area of the cellular anode to getter gas molecules within the apparatus and thereby reduce the gas pressure therewithin. While the pumping speed of such a unit was adequate for many uses there was a need for a pump having still greater pumping speed. One solution would be to scale up in size previously used pumps. The resulting scaled up pump would be awkward to use and difficult to construct due to its large diameter.

The present invention provides a novel getter ion vacuum pump wherein a plurality of anode members are interleaved with a plurality of cathode plates. The stack of interleaved members are suspended within the interior of an evacuable chamber and immersed in a strong magnetic field running substantially perpendicularly to the cathode plates. It has been found that a getter ion pump constructed in this manner has substantially enhanced pumping speed and is easy to fabricate, use and service.

The principal object of the present invention is to provide a novel getter ion vacuum pump having increased pumping speed which is relatively easy to build and service.

One feature of the present invention is the provision 3,231,175 Patented Jan. 25, 1966 of a stack of interleaved cellular anode members and planar cathode members, said cathode members being apertured in substantial alignment with the intersections of adjoining cellular compartments of said cellular anode members whereby the flow of gas through said members is facilitated, in use.

Another feature of the present invention is the provision of a perforated cathode plate, the perforated portions faciliating gas access therethrough and being disposed in substantial alignment and confined predominately to the marginal portions of the glow discharge passageways whereby portions of the perforated cathode which are not subject to intense ion bombardment are removed to facilitate gas access therethrough.

Another feature of the present invention is the provision of a perforated magnetic pole piece, the perforated portion thereof serving to facilitate gas access therethrough for enhancing the flow of gas into the glow discharge passageway and thereby enhancing the pumping speed of the pump.

.Another feature of the present invention is the provision of a magnetic pole disposed internally of the vacuum envelope for concentrating the magnetic field through the cathode and anode members whereby the size of the magnet may be minimized.

Other features and advantages of the present invention will become apparent upon a perusal of the specification taken in connection with the accompanying drawings wherein,

FIG..1 is a longitudinal cross section view of the novel getter ion vacuum pump of the present invention,

FIG. 2 is a cross sectional view of a portion of the structure FIG. 1 taken along line 2-2 in the direction of the arrows,

FIG. 3 is an exploded perspective view .of a portion of the structure FIG. '1 delineated by line 3--3,

FIG. 4 is an enlarged fragmentary view of an alternative embodiment of a portion of the structure of FIG. 1 taken along line 44 in the direction of the arrows,

FIG. 5 is a longitudinal cross sectional view of a novel getter ion vacuum pump of the present invention, and

FIG. 6 is a perspective view of a portion of the structure of FIG. 5.

Referring now to FIGURES 1-3 there is shown an embodiment of the present invention. More specifically, a plurality of cathode plates 1 having triangular shaped ears thereon are carried transversely of and longitudinally spaced apart along the length of two cathode support rods 2. The cathode plates 1 are made of a reactive material as of, for example, titanium or chromium. Other suitable reactive materials are molybdenum, tungsten, tantalum, niobium, ion, zirconium, nickel, barium, thorium, magnesium, calcium, strontium plus other transition elements of the fourth, fifth and sixth groups of longitudinally of the cathode support rods 2 via a plurality of hollow cylindrical cathode spacers 3, as of, for example, stainless steel.

The cathode spacers 3 are apertured at 4 to allow gases trapped within the spaces between the spacer and the cathode support rods 2 to easily escape during operation of the pump. The cathode support rods 2 are anchored at their end portions to upper and lower cross arms 5 via lock nuts 6. The cross arms 5 are made of a material which is structurally strong and able to withstand substantial temperatures as of, for example, stainless steel.

A plurality of cellular anodes 7 which may be fabr cated, for example, from titanium sheet metal approximately 0.015 inch thick spot-welded together are earned via four brackets 8 disposed on opposite sides thereof. The brackets 8 may be of, for example, titanium sheet metal spot-welded to the cellular anode 7 and the brackets 8 in turn are spot-welded to tubular anode spacers 9 as of, for example, stainless steel.

Two anode support rods 11 as of, for example, stainless steel are slideably inserted within the hollow anode spacers 9. The two anode support rods 11 are quadraturally placed with respect to the cathode support rods 2 and extend longitudinally of the stacked cellular anodes 7 and cathode plates 1. The anode spacers 9 are suitably apertured at 12 to allow gases within the space between the anode support rods 11 and the anode spacers 9 to be easily pumped during operation of the pump.

The ends of the anode support rods 11 are threaded to mate with the internal threads of frustr-ooonically shaped anode high voltage insulators 13 which in turn are carried from the upper and lower cross arms 5 via suitable screws 14. Hollow cylindrical anode insulator shields 15 are captured between the insulators 13 and anode spacers 9 at the ends of the anode support rods 11. The insulator shields 15 extend coaxially of and slightly spaced apart from the anode high voltage insulators 13 to prevent the condensation of disintegrated cathode material upon the anode high voltage insulators 13 and thus produce inadvertent shorting thereof.

A high voltage lead 16 is fixedly secured to the anode insulator shield 15 as by, for example, soldering and is clamped via clamp 18 at its other end to the center conductor 17 of a high voltage lead-in insulator assembly 19.

The stack of interleaved anodes 7 and cathode plates 1 is carried between the upper and lower cross arms 5 and is suspended within a hollow tubular envelope 21 as of, for example, stainless steel. The stacked assembly is carried from the tubular envelope 21 via two brackets 22 spotwelded to the inside surface of the envelope 21 substantially at the open end thereof. The upper cross arm 5 is fixedly secured to the two brackets 22 via two sheet metal screws 23.

The high voltage lead-in insulator assembly 19 includes the center conductor 17 as of, for example, stainless steel extending radially outward of the tubular envelope 21 through an aperture therein. A centrally apertured cup member 24 as of, for example, Kovar is fixedly secured substantially at the outermost extremity of the center conductor 17 as by, for example, brazing. A hollow cylindrical insulator 25 as of, for example, ceramic is disposed coaxially of the center conductor 17 and is fixedly secured at one end to the cup shaped frame member 24 as by, for example, a metal to ceramic seal. The ceramic insulator 25 is externally recessed at its other end and has affixed thereto and coaxially thereof an annular metallic frame member 26 as of, Kovar. An annular insulator shield 20 is carried transversely of the center conductor 17 for preventing disintegrated cathode material from condensing on the insulator 25. Frame member 2 6 is sealed to and closes off the open end of a hollow cylindrical adaptor 27 as of, for example, stainless steel. The hollow cylindrical adaptor 27 is carried from the inside periphery of the aperture in the side wall of the tubular envelope 21 as by, for example, a heliarc weld.

An electrical solenoid 28 is disposed concentrically of the tubular envelope 21 for producing a magnetic field directed axially of the interleaved stack of cellular anodes 7 and cathode plates 1. A hollow coolant tube 29 is wound in a helical fashion and alfixed to the inside peripheral surface of the solenoid 28 extending substantially the entire length thereof. The hollow coolant tube 29 extends radially from the inner helix therein to a position of larger diameter and is wound into another helix and sandwiched between two coaxial halves of the solenoid 28. The hollow coolant tube 29 is supplied with fluid coolant from a source, not shown.

A hollow cylindrical magnet yoke 31 as of, for example, iron circumscribes the solenoid 28 and is closed off at one end via a circular pole piece 32 as of, iron. The other end of the magnet yoke 31 is closed off via an annular pole piece 33 centrally apertured to receive the tubular envelope 21 slideably thereth-rough.

An annular flange 34 as of, for example, stainless steel is fixed-1y secured to the open end of the tubular envelope 21 as by, for example, a he'liarc weld. The flange 34 mates wit-h a similar flange member'35 carried upon the extremity of a hollow exhaust tubulation 36 communicating withthe apparatus it is desired to evacuate. The flanges 34 and 35 are provided with suitable rnating ridges thereon for compressing therebetween a soft metal gasket as of, for example, copper to assure a vacuum tight connection. The flanges 34 and 35 are pulled together via a plurality of bolts 37 spaced about the periphery of the flanges 34 and 35.

In operation the tubular envelope 21 is preferably exhausted to a pressure of at least 10- millimeters of mercury by, for example, a mechanical vacuum pump, not shown. A positive potential with respect to the potential of the cathode plates 1 as of, for example, 3 kv. is applied to the anode members 7 via center conductor 17 of the high voltage insulator assembly 19 and lead .16. The cathode plates 1 are electrically connected to the tubular envelope 21 which is preferably operated at ground potential. The solenoid 28 is energized with electrical current producing a magnetic field of approximately 1,000 gauss directed longitudinally of the interleaved stack of cellular anodes 7 and cathode plates 1.

When the operating potentials are applied a glow discharge is initiated between the interleaved anode and cathode members. The total glow discharge is subdivided by the anode cells or openings into a plurality of separated glow discharge columns, the discharge columns extending through the anode cells or glow discharge passageways which extend in the direction of the magnetic field. Positive ions created by the glow discharge are accelerated under the applied electric field between anodes 7 and cathode plates 1 and are caused to bombard the cathode plates 1 thereby disintegrating portions of the reactive cathode plates 1. The disintegrated portions of the cathode plates 1 diffuse within the interior of the tubular envelope 21 and condense upon the surfaces of the cellular anodes 7. Gas molecules coming in contact with the disintegrated cathode material are entrapped thereon and effectively removed from the gaseous state thereby reducing the pressure within the tubular envelope 21 and other structures communicating therewith.

After the pump has been in operation for a considerable period of time a substantial accumulation of atomized cathode material will have coated the cellular anodes 7 and a substantial proportion of the cathode plates 1 will have been eroded away due to the ion bombardment. The wornout cathode plates 1 may be easily replaced due to the unique construction of the present vacuum pump by removing the stack of interleaved cathodes 1 and anodes 7 and replacing the cathode plates 1 with new plates 1. At the same time the cellular anodes 7 may be replaced if required or merely sandblasted or chemically treated to remove the accumulation of cathode material. The elements are then reassembled and inserted within the tubular envelope 21.

It has been found that the pumping speed of a pump constructed in the above-described manner is substantially enhanced over the pumping speed obtainable with a single cellular anode 7 disposed between adjacent cathode plates 1. It has been found that the pumping speed of a getter ion pump is approximately proportional to the total cross sectional area of the cellular anodes 7.

The stack of alternately spaced anodes 7 and cathode plates 1 is spaced a substantial distance from the side walls of the tubular envelope 21 to facilitate the passage of gases therearound and into the spaces between the anodes 7 and cathode plates 1 to enhance the pumping speed.

The pumping speed of the apparatus (see FIG. 4) may be further increased by providing a plurality of apertures 38 in the cathode plates 1, said apertures 38 having their centers registering with the innersection of the vanes forming the cellular anodes 7. When the cathode plates 1 are apertured in this manner the transparency of the cathodes 1 to gases diffusing into the stack of interleaved anodes 7 and cathode plates 1 is substantially increased thereby enhancing the pumping speed of the apparatus. The amount of cathode material that is sputtered from the apertured cathode plates 1 is substantially the same as the amount of cathode material sputtered from the nonapertured cathode plates 1 because the cellular anode 7 has an ion focusing effect causing substantially all of the ion bombardment of the cathode plates 1 to take place in alignment with the central portion of the individual cells making up the cellular anode 7.

Referring now to FIGURES 5 and 6 there is shown another embodiment of the present invention. More specifically, this embodiment is similar to the structure shown in FIGURES 1-4 with the exception that the stack of alternately spaced anodes 7 and cathode plates 1 is carried from a flange assembly rather than from the inside of the tubular envelpe 21. In addition, permanent magnets employing a pole piece inside of the tubular envelope 21 have been utilized for supplying the magnetic field axially of the stack of anodes 7 and cathode plates 1.

The cathode support rods 2 carry the cathode plates 1 transversely thereof and longitudinally spaced apart via cathode spacers 3. Also carried transversely of the cathode support rods 2 is an apertured circular pole piece 41 as of, for example, iron. The ends of the cathode support rods 2 are threaded for mating with the internal threads of suitably placed bores in an annular flange 42 as of, for example, stainless steel which in turn is carried at the end of an exhaust tubulation 43. The cathode plates 1 are held on the cathode support rods 2 via nuts 44 threaded overthe free end portions thereof.

The anode support rods 11 are quadraturally spaced to the cathode support rods 2 and carry therefrom and transversely thereof the cellular anode members 7 via brackets 8 and anode spacers 9. The anode support rods 11 are anchored in anode high voltage insulators 13 which are carried within recesses provided in the pole piece 14 via screws 14. The cellular anodes 7 are captured on the anode support rods 11 via nuts 45 threaded over the free end portion of the anode support rods 11.

The positive operating voltage is applied to the cellular anode members 7 via the high voltage lead-in insulator assembly 19 and lead 16. The lead-in insulator assembly 19 is carried from the annular flange 42 and extends through a suitable aperture therein.

The pole piece 41 is provided with a plurality of apertures therethrough to facilitate the flow of gas into the stacked pumping structure. The pole piece 41 is closely spaced tothe side walls of the tubular envelope 21. A centrally apertured hexagonal plate 46 as of, for example, iron is carried from the tubular envelope 21 adjacent the pole piece 41 and forms a continuation of the pole piece 41. The fiat sides of the hexagonal plate 46 carry a 6 plurality of C shaped permanent magnets 47 as of, for example, alnico V. The C shaped permanent magnets 47 are secured at their ends via bolts to the flat sides of hexagonal plates 46 and 48 forming the pole pieces of the permanent magnets. Plate 48 is made of a magnetic permeable material as of, for example, iron.

Flange 34 which is fixedly secured to the tubular envelope 21 substantially at the open end thereof mates with flange 42 for compressing th'erebetween a soft metal gasket as of, for example, copper to assure a vacuum tight seal at the joint. The flanges 42 and 34 are held together via a plurality of bolts 37 spaced about the perimeter of the flanges.

The pump embodiment shown in FIGURES 5 and 6 has substantially the same mode of operation as the pump previously described with regard to FIGURES 14. However, the pump is simplified to the extent that the stack of alternately spaced anodes 7 and cathode plates 1 is supported directly from the flange 42. Moreover, the use of a current source and coolant tubes for the solenoid 28 is dispensed with. The magnetic field is provided by a plurality of permanent magnets 47 surrounding the envelope 21. The provision of the pole piece 41 internally of the tubular envelope 21 allows a more uniform magnetic field within the tubular envelope 21 and serves to concentrate the magnetic field through the anode 7.

Additional anodes 7 and cathode plates 1 may be added to the pump described in FIGURES 5 and 6 by extending the envelope 21 to accommodate the added members and by providing additional C shaped magnets positioned longitudinally of the envelope 21 and preferably provided with additional internal and external pole pieces 41 and 46 respectively as required.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the ac companying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. An electrical vacuum pump apparatus for evacuating an enveloped structure including means forming a cathode contained within the vacuum envelope for positive ion bombardment to remove matter in the gaseous state within the envelope and hence reduce the gas pressure in the enveloped structure, means forming an anode disposed within the envelope, means for applying a positive potential tosaid anode means with respect to said cathode means for producing a glow discharge therebetween and for bombarding said cathode means with positive ions produced in the glow discharge, means for producing and directing a magnetic field through said anode means, said anode means having a plurality of openings therein, the openings being distributed transversely to the direction of the magnetic field threading said anode and the openings of said anode means defining a plurality of glow discharge passageways extending in the same direction as the magnetic fi'eld for containing therein a plurality of separated simultaneous glow discharge portions in use, said cathode means including a pair of spaced apart portions disposed straddling said anode structure, and at least one of said cathode portions having perforated regions to facilitate gas access therethrough, said perforated regions of said cathode means being disposed in substantial alignment with and in mutually opposing relationship with and being confined predominantly to the margin portions of said glow discharge passageways whereby the cathode means portion mutually opposed to the axes of said glow discharge passageways and which is subjected to more intense ion bombardment remains intact while portions of said perforated cathode means which are not subject to such intense ion bombardment are removed to facilitate gas access therethrough.

2. An electrical apparatus as called for in claim 1 wherein said perforated cathode means is provided with a plurality of spaced apart perforated regions therein, said spaced perforated regions defining a plurality of gas access passageways through said cathode structure.

3. The apparatus according to claim 2 wherein said gas access passageways defined by said perforated regions of said cathode means have a typical transverse dimension of approximately one-half the transverse dimension of the typical glow discharge passageway in said anode.

4. The apparatus according to claim 2 wherein said glow discharge passageways in said anode are formed by a plurality of closely spaced cellular compartments.

5. The apparatus according to claim 4 wherein said perforated cathode means includes a perforated cathode plate provided with a plurality of spaced perforated regions forming the gas access passageways in said cathode plate, and said gas access passageways being substantially aligned, in the direction of the magnetic field, with the intersecting corners of said grouped cellular anode compartments.

6. An electrical vacuum pump apparatus for evacuating an enveloped structure including means forming a cathode contained within the vacuum envelope for positive ion bombardment to remove matter in the gaseous state Within the envelope and hence reduce the gas pressure in the enveloped structure, means forming an anode disposed within the envelope, means for providing a positive potential to said anode means with respect to said cathode means for producing a glow discharge therebetween and for bombarding said cathode means with positive ions produced in the glow discharge, means for producing and directing a magnetic field through said anode means, said anode means having a plurality of openings therein the openings being distributed transversely to the direction of the magnetic field threading said anode, the openings of said anode means defining a plurality of glow discharge passageways extending in the same direction as the magnetic field for containing therein a plurality of separated simultaneous glow discharge portions in use, said magnetic field producing and directing means including a pair of magnetic pole pieces disposed straddling said anode means, one of said magnetic pole pieces disposed within the vacuum envelope and being perforated to provide a gas access passageway therein, said gas access passageway communicating with said glow discharge passageways for enhancing the flow of gas into the glow discharge passageways and thereby enhance the pumping speed of the pump.

'7. The apparatus according to claim 6 wherein said perforated magnetic pole piece includes a plate having a plurality of perforated regions therein, said perforated regions in said magnetic pole piece providing a plurality of gas access passageways in gas communication with the glow discharge passageways in said anode structure.

8. The apparatus according to claim 6 wherein said cathode means includes a pair of substantially parallel directed cathode plates disposed straddling said apertured anode member, one of said cathode plates being perforated to define a plurality of gas access passageways therethrough, said cathode gas access passageways being disposed in substantial alignment, in the direction of the magnetic field, with the margins of said anode openings, and said apertured cathode plate being disposed inbetween said apertured pole piece and said apertured anode.

9. An electronic vacuum pump including an envelope member, spaced cathode plates and an anode disposed between said plates all disposed in said envelope member, a permanent magnet separate from and outside said envelope member and surrounding said anode and providing a magnetic field substantially perpendicular to the surface of the cathode plates, means within said envelope member and carried adjacent at least one of said cathode plates for concentrating the magnetic field, and means providing communication between the envelope member and associated equipment.

10. A vacuum pump comprising a cylindrical envelope member, a cathode plate spaced from one end of said envelope member, a hollow anode carried within said envelope member and defining an ionization space therein, and a second cathode plate spaced from the first cathode plate on the opposite 'end of said anode, magnetic means surrounding the exterior of said envelope member and providing a substantially axial magnetic field, means for concentrating the field and carried within the envelope member, and at least one outlet formed at the end of said envelope member and providing means for securing the pump to associated equipment.

11. Apparatus as in claim 10 wherein said magnetic means includes a magnet adjacent the envelope member, a pole piece disposed at said one end of said cylindrical envelope member, said pole piece including a portion extending upwardly and in contact with the adjacent portion of the envelope member.

12. An electronic vacuum pump including an envelope member, spaced cathode plates and a hollow anode disposed between said cathode plates all placed within said envelope member, a magnet outside said envelope member and surrounding said anode and providing a magnetic field substantially perpendicular to the surface of the cathode plates, means within said envelope member and carried adjacent each of said cathode plates for concentrating the magnetic field, and means providing communication between the envelope member and associated equipment.

13. A vacuum pump comprising a cylindrical envelope member, a cathode plate spaced from one end of said envelope member, an open anode carried Within said envelope member and defining an ionization space therein, a second cathode plate spaced from the first cathode plate on the opposite end of said anode, a high voltage lead-in extending through the envelope member and making electrical connection with the anode, magnetic means surrounding the exterior of said envelope member and providing a substantially axial magnetic field, means carried within the envelope member adjacent each of said cathode plates for concentrating the field within the anode, and a pair of axially disposed outlets at end of the envelope member.

14. A vacuum pump comprising a cylindrical envelope member, a cathode plate spaced from one end of said envelope member, an open anode carried within said envelope member, a second cathode plate spaced from the first cathode on the opposite side of said anode, magnetic means surrounding the exterior of said envelope member and providing a substantially axial magnetic field means carried within the envelope member for concentrating the field within the anode, at least one outlet disposed at one end of said envelope member,, and a high voltage lead-in disposed axially at the end of said envelope memher and providing means for making electrical connection through the envelope member to the anode.

15. An electronic vacuum pump including an envelope member, spaced cathode plates and an open anode disposed between said plates all disposed in said envelope member, magnetic means serving to provide a magnetic field substantially perpendicular to the surface of the cathode plates, means carried within the envelope member adjacent at least one of said cathode plates for concentrating the magnetic field with said anode, and outlet means for connecting the interior of the envelope member to associated equipment.

16. A vacuum pump comprising a cylindrical envelope member, spaced cathode plates and an open anode disposed between said plates all disposed in said envelope member, magnetic means serving to provide a field substantially perpendicular to the surface of the cathode plates, a high voltage lead-in extending through the envelope member and making electrical connection with the anode, means carried within the envelope member adjacent at least one of said cathode plates for concentrating the magnetic field Within said anode, and an outlet communicating with the interior of the envelope member and providing means for securing the pump to associated equipment.

17. An electronic vacuum pump including an envelope member, spaced cathode plates and an open anode disposed between said cathode plates all placed within the envelope member, means for providing a magnetic field substantially perpendicular to thesurface of the cathode plates, means carried within the envelope member adjacent at least one of said cathode plates for concentrating 10 the magnetic field within said anode, and a pair of outlet means communicating with the interior of said envelope member and providing means for connection to associated equipment.

References Cited by the Examiner UNITED STATES PATENTS 2,808,980 10/1957 Alpert 31373 10 DONLEY J. STOCKING, Primary Examiner.

GEORGE N. WESTBY, LAURENCE V. EFNER,

Examiners. 

1. AN ELECTRICAL VACUUM PUMP APPARATUS FOR EVACUATING AN ENVELOPED STRUCTURE INCLUDING MEANS FORMING A CATHODE CONTAINED WITHIN THE VACUUM ENVELOPE FOR POSITIVE ION BOMBARDMENT TO REMOVE MATTER IN THE GASEOUS STATE WITHIN THE ENVELOPE AND HENCE REDUCE THE GAS PRESSURE IN THE ENVELOPED STRUCTURE, MEANS FORMING AN ANODE DISPOSED WITHIN THE ENVELOPE, MEAN FOR APPLYING A POSITIVE POTENTIAL TO SAID ANODE MEANS WITH RESPECT TO SAID CATHODE MEANS FOR PRODUCING A GLOW DISCHARGE THEREBETWEEN AND FOR BOMBARDING SAID CATHODE MEANS WITH POSITIVE IONS PRODUCED IN THE GLOW DISCHARGE, MEANS FOR PRODUCING AND DIRECTING A MAGNETIC FIELD THROUGH SAID ANODE MEANS, SAID ANODE MEANS HAVING A PLURALITY OF OPENINGS THEREIN, THE OPENINGS BEING DISTRITUTED TRANSVERSELY TO THE DIRECTION OF THE MAGNETIC FIELD THREADING SAID ANODE AND THE OPENINGS OF SAID ANODE MEANS DEFINING A PLURALITY OF GLOW DISCHARGE PASSAGEWAYS EXTENDING IN THE SAME DIRECTION AS THE MAGNETIC FIELD FOR CONTAINING THEREIN A PLURALITY OF SEPARATED SIMULTANEOUS GLOW DISCHARGE PORTIONS IN USE, SAID CATHODE MEANS INCLUDING A PAIR OF SPACED APART PORTIONS DISPOSED STRADDLING SAID ANODE STRUCTURE, AND AT LEAST ONE OF SAID CATHODE PORTIONS HAVING PERFORATED REGIONS TO FACILITATE GAS ACCESS THERETHROUGH, SAID PERFORATED REGIONS OF SAID CATHODE MEANS BEING DISPOSED IN SUBSTANTIAL ALIGNMENT WITH AND IN MUTUALLY OPPOSING RELATIONSHIP WITH AND BEING CONFINED PREDOMINANTLY TO THE MARGIN PORTIONS OF SAID GLOW DISCHARGE PASSAGEWAYS WHEREBY THE CATHODE MEANS PORTION MUTUALLY OPPOSED TO THE AXES OF SAID GLOW DISCHARGE PASSAGEWAYS AND WHICH IS SUBJECTED TO MORE INTENSE ION BOMBARDMENT REMAINS INTACT WHILE PORTIONS OF SAID PERFORATED CATHODE MEANS WHICH ARE NOT SUBJECT TO SUCH INTENSE ION BOMBARDMENT ARE REMOVED TO FACILITATE GAS ACCESS THERETHROUGH. 